Gap Between OPT and H1B

I will be graduating in Dec. 2011 with an bachelor degree. I already got an offer from a company, but I can only start full-time in September. So I have a gap from Jan 2012 until September 2012. I'm planning to intern at the same company from Jan-Apr which I will be using my OPT. After that, I want to take graduate classes for 4 months without completion. But I know my OPT will be terminated when I start taking classes. So when I start in September as a full-time. Can I use my OPT again? Because unfortunately H1B will not be effective until October. If not, could anybody give me some advise, please?

When you re-enroll in a full-time program you will get a new I-20 from the college in which you've enrolled and will forfeit the rest of your OPT time.

You cannot begin working in September on the 2012 quota. How can you have an offer for a BS degree more than a year in advance? There is something not right about your story. Are you expecting to be hired by a company being started by a friend or family member or is it a legitimate US employer?